Transistor switching circuits



Jan. 28, 1958 R. L..BR|GHT ETAL l,2,821,639

TRANSISTOR SWITCHING CIRCUITS Filedocnza, 1954 W'TNESSES I i |NvENToRs Richard .|3r|gn| g George H. Royer B f 5W' Maw ATTORNEY TRANSISTOR SWITCHING CIRCUITS Richard L. Bright, Hempiield Township, Westmoreland County, and George H. Royer, Pittsburgh, Pa., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application October 28, 1954, Serial No. 465,322

Claims. (Cl. 307-885) When the load is of such a nature as to draw considerable current from the source, almost inevitably severe arcing has been encountered. Such arcing brings about a considerable amount of pitting and eroding of contact points which is detrimental to the life of the switches,

and additionally may bring on an explosion under uny favorable operating conditions in the atmosphere surrounding the switch.

It has also been known to use vibrating mechanical current interrupters for converting a unidirectional voltage to an alternating voltage having a rectangular waveform. Such devices, commonly known as choppers, have found extensive application in conjunction with thermocouples in heat control feedback circuits where it is necessary to provide an amplified signal in accordance with the magnitude of the unidirectional voltage output of the thermocouples. The relatively high operating frequency of mechanical choppers (typically 60 cycles per second) results in mechanical wear requiring considerable maintenance and replacement.

One broad object of our present invention is to provide p an arcless reversing switch.

Another broad object of our invention is to provide a reversing switch having no moving parts.

A further object of our invention is to provide a switch for reversing the polarity of a D. C. voltage having no moving parts that carry relatively heavy current.

A still further Object is to provide an all-electrical device for converting a direct voltage to a rectangular wave alternating voltage.

Yet another object is to provide a reversing switch for coupling a voltage source to a load wherein the terminals of the load are at no time open circuited during the reversing operation.

Other objects and features of the present invention will become apparent upon consideration of the following detailed description when taken in connection with the accompanying drawings, in which:

Figure 1 is a schematic diagram of a preferred embodil ment of our invention;

Fig. 2 is a schematic drawing of another embodiment of our invention, useful particularly when polarity reversals are required only at infrequent intervals; and

Fig. 3 is a schematic diagram of still another embodikment of our invention, likewise particularly useful when polarity reversals are required only at infrequent-inter` nited States Patent O HCC vals, wherein only static switching components are utilized.

In our copending application Serial No. 420,904, filed April 5, 1954, there is described a type of switching device essentially comprising a junction type transistor, having a reversible-polarity voltage source connected between the base and one of the other electrodes of the transistor. As described in the mentioned application, a junction transistor having a properly polarized potential source coupled between emitter and collector tends to accumulate charged particles in the portion of the base in the vicinity of the adjoining electrodes. These charged particles tend to neutralize a contact potential that normally exists at the juncture of a p type conducting region and n type conducting region. By draining these charges from the base, it has been found that current conduction between emitter and collector is almost entirely cut oli, the contact potential being superimposed upon the potential of the base region to erect an extremely eliective barrier to current ow. For a p-n-p type transistor, the neutralizing charges can be removed by connecting a voltage source between base and emitter with the positive terminal thereof connected to the base.

It is further noted in the aforementioned application that the emitter-collector current-saturation of a p-n-p junction type transistor can be achieved by coupling the base to one adjoining electrode by means of a potential source polarized in such a manner that the base is at a negative potential with respect to either of the adjoining electrodes. Similarly, an n-p-n type junction tran sistor can be brought to current saturation by connecting the base thereof to one of the adjoining electrodes so that the base is at a positive potential with respect to either adjoining electrodes.

In one aspect of our invention, a transistor switch element of the type described above is included in each of the legs of a 4terminal, 4-legged bridge circuit. Across opposing terminals of the bridge are connected a D. C. source and a Z-termnal load, the D. C. source serving in a dual capacity to furnish power to the load and to provide emitter-collector potentials for the various transistors in the bridge. Control voltages conveniently derived from the same source so as to be of the same frequency as either in phase or out of phase with each other, are coupled between the base and one of the other electrodes of each of the transistors so that one pair of non-adjoining legs of the bridge is conducting and the other pair is non-conducting. The electrical connection between the load terminals and the source terminals thereof, reversed with each alternation of the vsupply voltage, produces the same eifect as a cross-connected double-pole, double-throw switch.

Referring to Fig. l of the drawings, there is shown a D. C. source 101 connected to a load 102 by means of a bridge circuit 103 including p-n-p type transistor switch elements 104, 111, 119, and 127. Source 101 is pictured as a battery, but may be a thermocouple or other appropriate unidirectional potential source. Load 102 may be the input circuit of an alternating` current amplifier in those applications wherein source 101 is a thermocouple. Source 101 is connected across bridge terminals 139 and 143 so that terminal 139 is at a positive potential with respect to terminal 143. Load 102 is connected between terminals 137 and 141 of the bridge.

The emitter 109 of transistor 104 is connected to terminal 137, and the collector 107 thereof to terminal 143. Similarly, the emitter 131 and collector 13S of transistor 127 are respectively connected to bridge terminals 141 and 143. Terminal 139 is connected to terminals 137 and 141 through the transistors 111 and 119 respectively. In this latter connection, emitters. 115. and

125 of transistors 111 and 119 are both connected to terminals 139, whereas collectors 117 and 121 thereof are conencted to terminals 137 and 141 respectively.

Control voltage source171 is .preferably a rectangular wave generator, such asa controlled frequency. multivibrator, operable to drive the transistors from cut-o to a condition of emitter-collector current saturation and viceV versa, without an appreciable intermediate period of class Aoperationthat could damage the transistors. The output of control source 171V is connected to the bridge 103 by meansof transformer 145. This transformenwhich should be capable of passing audio frequency so as to transform a square4 wave withreasonable fidelity, has a primary winding 147 directly connected to-control voltage source 171, and a plurality of secondary windings 149, 151, 153, and 155. The output terminals 157 and, 159 of windingV 149 are respectively connected to. bridge load terminal 137 and. to base 105 of transistor` 104. Similarly, terminals 163 and 1610i windingv151 arerespectively joined to bridge load terminal,141 andv to base electrode 129A ofY transistor 127. Windings 153' and 155 have a common terminalY 167 connected to bridge input terminal 139; the other terminall165 of winding 153 is directly `connected tothe base electrode 123 of'transistor 119, and terminal 1'69.` of winding 155`is similarly connected to base electrode 110 of transistor 111.

The operation of this embodiment of our invention is as follows. Assuming that control voltage source 171 is on a half cycle ofoperation such that terminals 159, 163, 165, and'167 are respectively positive with respect to terminals 157, 161, 167, and.' 169, current will flow from emittery to collector of transistors 111 and 127, inasmuch as. the'base electrodes thereof are at a negative potential with respect to their respective emitters. There will be noi current'iiow through transistors 104 and 119 because their'baseelectrodes are at apositive potential with respect to both their respective emitters and collectors'. Therefore, source terminals 139 and 143 will be'respectively connected to load terminals 137 and 141, and terminal 137 will be at` a positive potential with respect to terminal 141.

On the next half cycle of operation, the conduction states of the transistors will be reversedy since the potentials of the bases thereof relative to their respective emitters are reversed. Under this situation, load terminal 141 will be connected to source terminal 139 and load'terrninal 137 to source terminal 143. The polarity ofterminal 141 relative to terminal 139 will, therefore, be the reverse of the polarity which is obtained over the first halfcycleof operation.

The embodiment just described cannot readily be adapted for use in those applications wherein only infrequent'polarity reversals across the load are required, such as when the load is the armature of a D. C. motor. In Fig. 2, there is shown an embodiment of our invention that is suitable for use under such circumstances. Reference numerals in Figs..l and 2, wherein the last two digits correspond, designate the same components as in Fig. l; for example, D. C. source 101 and load 102 are the same as source 201 and load 202 of Fig. 2. Bridge circuit 203 includes transistors 204, 211, 219, and 227 connected to load terminals 237 and 241 and'to source terminals 243 and 239 in the same manner as described above. In this embodiment, however, transistors 204 and 227 are of the n-p-n typerather than the p-n-p type of transistor utilized as circuit elements 104 and 127. The emitters of transistors 204 and 227 are connected to source terminals 243, and the collectors thereof are respectively connected to load'terrninals 237 and 241. The emitters and'coliectors of transistors 211 and 219 are connected` to the bridge terminals 237, 239, and 241 in thel samemanner-as transistors 111 and`119.

Control ".voltagesxfor they transistors 204 .and 227 are derived from'. batteriess279. and 231,'. and. the Vcontrol voltage for transistors 211 and 219 are derived from batteries 233 and 285.' To insure current cut-oi, the voltage output of batteries 281 and 285 must be sufcient to insure that the bases of the n-p-n and p-n-p transistors are respectively at a positive and negative potential with respect to their other electrodes. The voltage of batteries 279 and 283 need only be sufficient to insure current saturation.

All. oiV the batteries are connected to the bases of the transistors through a control relay 274, the actuating coil 272 of which is energized by D. C. source 276 connected across the coil by S. P. S. T. switch 273. ln itsnonenergized position, the relay closes contact members 27721,l 277C, 277e, and 277g; when the actuating coil 272 is energized, contact members277 b, 277a', 277 f, and 277k are closed. Batteries 279 and 281 are connected in seriesadding, with the juncture thereof joined to bridge supply terminal 243. The base electrodes of transistors 204 and 227 are connected to the positive terminal of battery 279 through contact members 277b and 277e respectively, and to the negative terminal of battery 281 through contact members 277a and 27761 respectively. In similar manner, batteries 283 and 235 are connected in series-adding with the juncture thereof, directly connected. to the other supply terminal 239. Likewise, the base electrodes of transistors 211 and 219 are coupled to the positive terrrn'nal of battery 285 through contact members 277k4 and 277e.respectively, and to the negative terminal of'the battery 283 through contact members 277g and`2777 respectively.

When actuating coil 272 is not energized by battery 276l (i. e., when switch 273 is open) transistors 227 and 211' willf be conducting'while transistors 204 and 219 will be non-conducting. The negative supply terminal 243 will, therefore, be connected to load terminal 241 while positive supply terminal 239 will be connected to load terminal 237. Upon closure of vswitch 273, transistors 204 and 219 will become conducting, while transistors 227 and 211V will become non-.conducting. Load terminals 241 and 237 will respectively be connected to supply terminals 239 and 243 so as to reverse the polarity of the voltage appearing across the load.V

InFig. 3, there is shown an embodiment of our invention quite similar to that shown in Fig. 2 wherein the ynecessity for the multi-contact switching relay requiredin. theembodiment of Fig. 2 is entirely eliminated. Reference numerals in Figs. 2 and 3, wherein the last two digits correspond, designate the same components as have been described with reference to Fig. 2.

This. embodiment of our invention utilizes a pair of p-n-p type junction transistors 326- and 356 and a pairyof n-p-n type junction transistors 334 and 346'wherein the bases 330 and 338 of transistors 326 and 334 respectively are connected to a control voltage terminal 363l and the bases 360 and 350 of transistors 356 and 346 are tied to a control voltage terminal 366. The control voltage terminals 366, 363 may be connected to any convenient source of reversible-polarity control voltage 371, such as is well known inthe art. A plurality of potential sources306, 30S, 310, and 312 which may be batteries or other convenient direct current sources, are connected-inseries-adding in the. order named. The positive. terminalY of source 306 and the negative terminal of source 308 are connected to terminal 343 of bridge 304, which terminal is further connected tothe emitters: of..n,-pn. junction transistors 303 and 327, as has been described with reference to Fig. l. Likewise, bridge terminal 339 is connected to thepositive terminal 1310 andsthe'negative terminal of source 312. The emittersq332, 336, 352, and 35S of transistors 326, 334, 346, and'y 356 respectivelyk are all connected' together and toutheA positive terminal of source 308 and4 the lnegative@terminal of source 310.v` Collector'V electrode 362 of transistor 356 is connected to the base electrode of transistor 327 through resistor 364 and collector electrode 328 of transistor 326 is connected to the base electrode of transistor 303' through resistor 324. The base electrodes of transistors 303 and 327 are additionally connected to the negative terminal of source 306 through resistors 318 and 320, respectively. Likewise, the collector electrode 348 of transistor 346 is connected to the base electrode of transistor 319 through resistor 344, and collector electrode 340 of transistor 334 is connected to the base electrode of transistor 311 through resistor 342. The base electrodes of transistors 311 and 319 are further connected to the positive terminal of source 312 through resistors 316 and 314, respectively. As has been described with reference to Fig. 2, the collector electrodes of transistors 303 and 311 are connected to bridge terminal 337 and to one terminal of the load; the collector electrodes of transistors 327 and 319 are connected to bridge terminal 341 and to the other terminal of load 302.

Let it be assumed that terminal 368 is positive with respect to terminal 366. In this circumstance, transistors 326 and 346 will be biased to cut olf, whereas, transistors 356 and 334 will be biased to conduction. The base electrode of n-p-n junction transistor 327 will be biased positively relative to its emitter (i. e., to conduction) by means of D. C. source 308 through resistor 364 and the emitter to collector current conduction path of transistor 356. The base of p-n-p junction transistor 311 will be biased negatively with respect to its emitter by means of D. C. source 310 through transistor 334 and will likewise be driven to saturation. Transistors 303 and 319 will be biased to cut-off by means of batteries 306 and 312, respectively, resistors 318 and 316 isolating transistors 303 and 319 from the eiect of transistors 326 and 346 being rendered conducting. Load terminal 337 will thus be at a positive potential with respect to terminal 341.

With the control voltage terminal 366 positive with respect to terminal 368, transistors 326 and 346 will be biased to collector current saturation and transistors 334 and 356 will be cut-oft. The base of p-n-p junction transistor 319 will be biased negatively with respect to its emitter by means of D. C. source 310 through transistor 346 and resistor 344, and the base of transistor 303 will be biased positively with respect to its emitter by means of source 308 through transistor 326 and resistor 324; thus, transistors 319 and 303 will be driven to saturation. Transistors 327 and 311 will simultaneously be biased to cut-oil by means of batteries 306 and 312, respectively. Therefore, terminal 341 will be positive with respect to terminal 337 and the opposite to that polarity which was obtained with the control voltage polarity described as above. In this instance, resistors 314 and 320 will isolate the base circuits of transistors 311 and 317 from the effects of transistors 326 and 346 being rendered conducting.

Thus, it can be seen that by the teaching of our invention there is produced a reversing switch having no moving parts that carry heavy current. The embodiment of our invention depicted in Fig, 1 needs no moving part whatsoever and is particularly adapted for uses requiring an alternating rectangular-wave output voltage of uniform frequency, the amplitude of which is functionally related to the amplitude of the D. C. source voltage. Inasmuch as the source is at all times coupled to the load by means of circuit elements incapable of generating an arc, there is no danger of an explosion being initiated thereby even under the most unfavorable operating conditions.

Although the embodiments disclosed in the preceding specification are preferred, other modifications will be apparent to those skilled in the art which do not depart from the scope of the broadest aspect of the-present invention.

We claim as our invention:

l. Apparatus for connecting a 2-terminal unidirectional voltage source to a pair of load terminals comprising: first and second p-n-p junction transistors the emitters of which are connected to a iirst source terminal and the collectors of which are respectively coupled to first and second load terminals; first and second n-p-n junction transistors, the emitter electrodes of which are coupled to a second source terminal and the collector electrodes of which are respectively connected to said rst and second load terminals; means for simultaneously applying a potential of one polarity between the emitter and base electrodes of said lrst p-n-p transistor and of said rst n-p-n transistor, and a potential of the opposite polarity between emitter and base electrodes of said second p-n-p transistor and said second n-p-n transistor; and means for simultaneously reversing the polarity of the potential applied between each of said emitter and base electrodes.

2. Apparatus for connecting a unidirectional voltage source to a pair of load terminals comprising: a first pair of transistor means of opposite conductivity type each having at least emitter, base and collector electrodes coupling said source to said load terminals through the emitter-to-collector current conduction path thereof so that a voltage of one polarity appears thereacross; a second pair of transistor means of opposite conductivity types each having at least emitter, base and collector electrodes coupling said source to said load terminals through the emitter-to-collector current conduction path thereof so that a voltage of the opposite polarity appears thereacross, and control voltage means coupled between said base and adjacent electrode of each of said transistor means adapted to render said first and second pairs of transistor means conducting in alternation.

3. Apparatus for connecting a unidirectional voltage source to a pair of load terminals comprising: a frst pair of transistor means of opposite conductivity type each having at least emitter, base and collector electrodes coupling said source to said load terminals through the emitter-to-collector current conduction path thereof so that a voltage of one polarity appears thereacross; a second pair of transistor means of opposite conductivity types each having at least emitter, base and collector electrodes coupling said source to said load terminals through the emitter-to-collector current conduction path thereof so that a voltage of the opposite polarity appears thereacross, and control voltage means coupled between said base and adjacent electrode of each of said transistor means adapted to render said first and second pairs of transistor means conducting in alternation, said control voltage means including said voltage source.

4. Apparatus for connecting a pair of unidirectional voltage input terminals to a pair of load terminals comprising: a first pair of transistor means of opposite conductivity type each having at least emitter, base and collector electrodes coupling said input terminals to said load terminals through the emitter-to-collector current conduction path thereof so that a voltage of one polarity will appear thereacross; a second pair of transistor means of opposite conductivity types each having at least emitter, base and collector electrodes coupling said input terminals to said load through the emitter-to-collector current conduction path thereof so that a voltage of the opposite polarity will appear thereacross, first potential source means coupled between base and an adjacent electrode of each of said transistor means adapted to bias said transistor means to cut-off; and means including second potential source means coupled between base and an adjacent electrode of each of said transistor means adapted to selectively bias said first and second pairs of transistor means to conduction.

5. Apparatus for connecting a pair of unidirectional voltage:A inputterrrnals.: to af pair of loadtermualsfcomprising: a rst pair of transistor means of oppositeA conductivity type each havingzatleast: emitter, basez and col-lector: electrodes couplingsaid inputfV terminals to said loa'dl terminals through the emitter-to-collector current conduction-path thereof so that a voltage of one polarity will appear thereacross; .a second pairvof transistor means ofopposite'conductivity types eachlhavng atleast emitter, baserandicollector electrodes coupling said input tere Initials: to: said: lo ad through Y the emitter-to-collector' current conduction path thereof softhat a voltage of the opposite'A polarity willzappear'thereacross, rst potential source means.y coupled betweenbaseand an adjacent electrodeV of each1of` said-'transistor means adapted toi bias said transistor means toV cut-olf; thereafter'second potential source 'means adapted to :be Ycoupled between base electrode of: eaclllof: said transistormeans and the adjoining 1 electrode theretorconnectedwto one 1 of said` input terminals,` ,so as tobiassaid transistor means to collector current saturation, a third pair of* transistor means of opposite' conductivity typeseach having atleastemitte'r; baseY and collector electrodes adapted to couple saidsecondpotential. s'ourceemeans'to said rst pairof transistor means throughvthe Yernitte'r-to#collectorl current conductionpaths thereof; afourthlpairl of transistor means of oppositev conductivityA types'eachhaving at least emitter, base and collector electrodes adapted'to couple said seeond-vpotentialrsource means to said second'pair of transis# tor means-through the e'r'ni'tter-to-collector current'con# duction path thereof; and means` adapted to drive said third and fourth-pairsof transistor means to collector current saturation in alternation.

References Cited inrthele of this patent STATES PATENTS 

